Is this perk only effective vs Humans/humanoids?
Where would you hit other animals? Like a giant radscorpian or a Radroach for the effect

Pretty sure there's a "torso" for just about everything in New Vegas, even Radscorpions and Insects, so it should affect everything.

Edit: Just look for the the big part of an enemy where the head connects. That'll probably be the VATS limb you want to hit.

It does work on non humanoid enemies you just aim at the equivalent of their torso